1. Field of the Invention
The present invention relates to a method for producing a mask ROM and a photomask used for such a method.
2. Description of the Related Art
A mask ROM of the type to which the present invention relates is a semiconductor memory, on which data is written by placing a resist over a plurality of memory cells arranged in a matrix, exposing the resist to light via a photomask so as to pattern the resist, and then selectively processing the memory cells. The mask ROM may, in some cases, form a part of a semiconductor device.
FIG. 10 illustrates an example of a photomask, where the white portions block light while the black (hatched) portions transmit light. In this photomask, memory cell regions 101 respectively correspond to the memory cells of the mask ROM while slits 102 correspond to boundaries between the memory cells.
Each of the memory cell regions 101 is slightly smaller than a memory cell of the mask ROM and is formed to occupy the inside of the memory cell and to form the slits 102 between the memory cell regions 101. Such an arrangement prevents light, which is supposed to be transmitted through the memory cell region 101 and to irradiate a portion of the resist over the corresponding memory cell, from leaking to and influencing the adjacent memory cells.
For example, in the case where data is written by selecting memory cells with a resist pattern and implanting an impurity into the selected memory cells, if light leaks out of the selected memory cells to adjacent memory cells, thereby disordering the resist pattern, the impurity may diffuse out of the selected memory cells into the adjacent memory cells. In such a case, an error occurs in the data to be written, thereby leading to performance abnormality of the mask ROM and lowering the production yield thereof.
For example, when the length of a side of the memory cell region 101 is about 0.8 to 0.9 .mu.m, the width of the slit 102 is set to about 0.4 .mu.m, so as to avoid the influence of light leaking to adjacent memory cells.
However, light leaking out of the memory cell region 101 consequently reduces the width of a linear portion of the resist pattern corresponding to the slit 102. In such a case, the linear portion may be collapsed or stripped off. The collapsed or stripped linear portion leads to circuit defects. This problem is more prominent when the slits 102 are provided in a parallel stripe pattern than when provided in a matrix as illustrated in FIG. 10.
On the other hand, when the width of the slit 102 is increased in order to avoid the above-described problem, the total area of the slits 102 increases. This makes it difficult to increase the density of memory cells and to downsize a mask ROM, thereby increasing the production cost thereof.
In view of this, an arrangement such as that illustrated in FIG. 11 has been proposed (see Japanese Laid-Open Publication No. 6-13250). In this arrangement, memory cell regions 111 for transmitting light are connected to one another so as to eliminate the slits between the memory cell regions 111, while memory cell regions 112 for blocking light are connected together so as to eliminate slits between the memory cell regions 112.
However, when a resist pattern illustrated in FIG. 12 is formed by using the photomask illustrated in FIG. 11, the four corners of each covering portion 113 and 113' of the resist pattern corresponding to the light blocking portions 112 and 112' of the photomask will be rounded off due to the optical diffraction occurring at the four corners of the light blocking portions 112 and 112' of the photomask. In such a case, light may leak to and influence the memory cells under the covering portions 113 and 113', thereby generating data errors.
In an attempt to prevent the corners of the resist pattern from being blunt and rounded off, another arrangement has been proposed (see Japanese Laid-Open Publication No. 7-147334). In this case, as illustrated in FIG. 13, an auxiliary pattern 123 is provided at each corner of a light transmitting portion 122 formed of light transmitting memory cell regions 121.
However, when such an auxiliary pattern 123 is used, the amount of information required to represent a photomask pattern increases because the pattern is then defined by smaller units, whereby the number of addresses designating the units increases. This significantly increases the data processing time. Moreover, it will also require fine processing of the resist, making the production thereof difficult, and thereby increasing the cost of a photomask as well as the cost of a mask ROM.
Moreover, although the auxiliary pattern 123 is favorable for the light transmitting portion 122, it is not favorable for a part 124 of the light blocking portions adjoining the light transmitting portion 122, and does not prevent this part 124 from being deformed. That is, such an auxiliary pattern is not appropriate for improving both the resolution of the light transmitting portions and the resolution of the light blocking portions of a photomask at once. In order to improve both of the resolutions at once, the number of types of the auxiliary pattern needs to be increased, which further complicates the photomask pattern.